Lubricating oil in the compressor of a heating, ventilation and air conditioning (HVAC) system provides lubrication for moving parts in the compressor. Good lubrication ensures the safe operation of the compressor. For a refrigerant compressor, the oil lubricating capability decreases when the oil is mixed with liquid refrigerant. For example, this may happen when the defrost operation is turned on during the heating season, since under such conditions, the indoor fan is typically shut down, and liquid in the evaporator may not be evaporated. As a result, large amounts of liquid refrigerant may enter the compressor chamber and mix with the lubricating oil.
To quantify how much liquid refrigerant is mixed with oil in the compressor, an important index under investigation is oil concentration. For reliable operations, oil concentration needs to be above a certain level such that the viscosity of the oil/refrigerant mixture is large enough to guarantee sufficient lubrication for moving parts in the compressor.
All refrigerant compressors circulate some amount of oil through the system. It is essential that oil be returned in the system. However, in an evaporator, when superheat is large and evaporating temperature is low, oil viscosity may become high because liquid refrigerant becomes vapor in the superheat range. If vapor velocity is not sufficient to transport the oil, some oil may remain in the evaporator. Similarly, in suction lines, oil retention may be a problem if refrigerant vapor velocity is not sufficient or the refrigerant temperature is low.
For a multi-evaporator system with a vertical gas line, if the vapor velocity is not high enough, the oil cannot be pushed upward and return to the compressor. When a significant amount of oil remains in the evaporator-condenser-gas line circuit or accumulator, the oil in the compressor will be not sufficient to provide reliable lubrication.
Conventionally, the amount and concentration of oil in the compressor cannot be directly measured without special sensors. For purposes of research and development on the system, special designs can be used to place costly viscosity sensors at the bottom of the compressor to measure the viscosity of the oil/refrigerant mixture in the compressor, and oil concentration is calculated from the value of viscosity and oil temperature. Through a glass window installed at the side of compressor, the oil/refrigerant mixture liquid level can be measured. Without a viscosity sensor or a special oil concentration meter that is not available in actual application of air conditioning and refrigeration systems, the amount and concentration of oil in the compressor cannot be determined in conventional systems.